The present invention relates to the general field of undersea pipes resting on the sea bottom or providing a bottom-to-surface connection for transferring hydrocarbons, in particular oil and gas, coming from undersea production wells.
The invention relates more precisely to coaxial undersea pipes of the pipe in pipe (PIP) type, where an inner tube transports the fluid and an outer tube coaxial around the inner tube, and also referred to as the “outer envelope”, is in contact with the surrounding medium, i.e. water. The annular space between the two tubes can be filled with an insulating material or it may be evacuated of any gas in such a manner as to provide thermal insulation for the fluids flowing in the inner tube. Such coaxial undersea pipes are used particularly for installations at great depths where the water temperature is typically 4° C.
In general, these coaxial pipes are assembled on land to constitute elements of unit length (known as double, triple, or quadruple joints, and referred to below as “quad-joints” for quadruple sections of tube), having length of the order of 10 meters (m) to 100 m depending on the loading capacity of the laying system. Such strings of pipes are thus transported to sea on a laying vessel.
During laying, the quad-joints are connected to one another on board the vessel while they are being laid at sea. Laying may be performed via a J-lay tower positioned on the laying vessel. With J-laying, the undersea pipe is typically lowered from the laying vessel practically vertically (at an angle in the range +30° to −10° relative to the vertical). J-laying comprises single-catenary laying in which the quasi-vertical angle of inclination of the pipe diminishes progressively on going down towards the bottom where it matches the slope of the sea bottom.
The J-laying method makes it necessary to move each quad-joint from a horizontal position (along the deck of the laying vessel) to a vertical position in order to align it with the J-lay tower. The quad-joint is held vertically by the laying tower in order to enable its bottom end to be welded to the top end of the pipe. Once the quad-joint has been welded to the pipe, the quad-joint is lowered into the sea while moving the laying vessel forwards by an amount corresponding to the length of the quad-joint.
Applied to coaxial PIP type pipes, the J-laying method makes provision for the quad-joint to be welded vertically to the undersea pipe already in place. The new quad-joint is presented offset upwards between the outer tube and the inner tube in order to enable the inner tube to be welded to that of the undersea pipe waiting in the J-lay tower holding device, then to slide the outer tube of the quad-joint relative to the inner tube downwards so as to weld its bottom end to the corresponding free end of the outer tube of the undersea pipe. Said pipe is then lowered into the sea and the operation is repeated with a new quad-joint.
In practice, such a laying method presents a certain number of problems. In particular, during hoisting of the new quad-joint, the inner tube must be held in position to prevent it slipping freely. Until now, complex special devices have been placed at the end to reinforce such holding, prior to being removed after the inner tube has been welded to the undersea pipe. However, using such devices is costly in terms of laying time. In addition, during laying of the inner tube of the quad-joint on the inner tube of the undersea pipe waiting vertically in the J-lay tower, the inner tube, if it is free, tends to deform helically in the outer tube of the undersea pipe, in such a manner that it is then generally necessary to adjust the top portion of the pipe in order to take up the clearance (the inner tube is cut as much as necessary). Finally, the step of sliding the outer tube relative to the inner tube risks leading to certain parts positioned around the inner tube of the quad-joint being pulled off (such as thermal insulation, heating cables, optical fibers, centering spacers, etc.).